1. Field of the Invention
The present invention relates to a laser annealer employing a laser beam. The present invention also relates to a semiconductor device manufacturing method for manufacturing a semiconductor device through steps including a step using a laser annealing method. The semiconductor device means herein one of a general device which can function by employing semiconductor characteristic and which involve an electro-optic device such as a liquid crystal display and a light emitting device, and an electronic device which has the electro-optic device incorporated therein as a component.
In recent years, study has been broadly made on the art to carry out laser anneal on a semiconductor film formed over an insulating substrate of glass or the like in order for crystallization or improving crystallinity. Such semiconductor films often use silicon. In the present description, the means for crystallizing a semiconductor film by using a laser beam and obtaining a crystalline semiconductor film is referred to as laser crystallization.
The glass substrate is cheap in price and excellent in workability as compared to the conventionally often used synthetic quartz glass substrate, having a merit to easily prepare a large-area substrate. This is the reason of the studies noted above. Meanwhile, the laser is used, by preference, in crystallization because the glass substrate is low in melting point. The laser can deliver high energy only to the semiconductor film without substantially increasing in substrate temperature. Furthermore, throughput is by far high as compared to the heating means using an electric furnace.
Crystalline semiconductor films are formed from many crystal grains, and therefore they are also referred to as polycrystalline semiconductor films. Because the crystalline semiconductor film formed through laser anneal has high mobility, thin film transistors (TFTs) can be formed using the crystalline semiconductor film. They are broadly utilized, e.g. in a monolithic liquid-crystal electrooptical device having pixel-driving and drive-circuit TFTs formed on one glass substrate.
Meanwhile, there is preferential use of a method for laser anneal that the high-output pulse laser light, of an excimer laser or the like, is formed through an optical system into a square spot in several-centimeter square or a linear form having a length of 10 centimeters or longer on an irradiation plane in order to scan the laser light (or moving a laser-light irradiation position relatively to the irradiated plane), because of high producibility and industrial superiority. By the way, to form the laser light to linear shape means that the laser light is formed to linear shape at the irradiated plane. It means that the cross-sectional shape of laser light forms linear shape. Further, the “linear shape” referred herein is not strictly means for “line” but means for the rectangular which aspect is large or oval shape. For example, the aspect ratio is 10 or more. (Preferably 100-10000).
Particularly, the use of a linear beam can realize laser irradiation over the entire irradiation surface by scanning only in the direction perpendicular to a lengthwise direction of the linear beam, differently from the case using the laser light in a spot form requiring scanning, providing high production efficiency. The scanning in a direction perpendicular to the lengthwise direction is carried out because the direction of scanning is the highest in efficiency. Due to the high production efficiency, the use of a linear beam formed of pulse-oscillated excimer laser light through a proper optical system in the current laser anneal process is in the mainstream of the technology to manufacture liquid crystal display devices using TFTs.
2. Related Art
To form an excimer laser beam, KrF (wavelength: 248 nm) or XeCl (wavelength: 308 nm) is used as excitation gas. However, such gases as Kr (krypton) and Xe (xenon) are quite expensive. Due to this, if Kr or Xe is employed and gas exchange is conducted more frequently, manufacturing cost is disadvantageously pushed up.
In addition, attachments such as a laser tube for laser oscillation and a gas purifier for removing unnecessary compounds generated during oscillation are required to be replaced once in two or three years. Most of these attachments are expensive, which again disadvantageously pushes up manufacturing cost.
As described above, a laser irradiation device employing an excimer laser beam exhibits high performance. However, the laser irradiation device of this type takes much labor for maintenance and the running cost (which means herein cost entailed by the operation of the device) thereof is disadvantageously high if used as a mass-production laser irradiation device.
Therefore, to realize a laser lower in running cost than the excimer laser and a laser annealing method employing the laser, there is proposed a method of using a solid-state laser (which outputs a laser beam with a crystal rod set as a resonant cavity).
Using a YAG laser which is one of the typical solid-state lasers, a laser beam is irradiated to a semiconductor film. According to the YAG laser, a laser beam (wavelength: 532 nm) which is modulated to a second harmonic by a nonlinear optical element is processed into a linear beam, which has a linear shape on an irradiation surface, by an optical system. The semiconductor film is an amorphous silicon film which has a thickness of 55 nm and which is formed on a substrate (“1737 substrate” manufactured by Corning Inc.) by a plasma CVD method. However, on a crystalline silicon film obtained by executing steps including a step using a laser annealing method to the amorphous silicon film, a concentric pattern is formed. This pattern indicates that the material property of the crystalline silicon film in the plane is not uniform. Due to this, if a TFT is manufactured using a crystalline semiconductor on which a concentric pattern is formed, the pattern adversely influences the electrical characteristic of the TFT. Here, a pattern of concentric circles is described as concentric pattern.
Further, as the screen of the electro-optic device is made large in size, the area of mother glass increases. Following this, demand for irradiating a laser beam to a semiconductor layer provided on the mother glass serving as a substrate at high rate rises for the laser annealing method.
In addition, demand for compensating for the poor power of a laser beam which is employed to temporarily melt a semiconductor layer in the crystallization of the semiconductor layer rises for the laser annealing method.